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<h1 align="center">SQL As Understood By SQLite</h1><p><a href="lang.html">[Top]</a></p><h2>UPDATE</h2><h4><a href="syntaxdiagrams.html#update-stmt">update-stmt:</a></h4><blockquote> <img alt="syntax diagram update-stmt" src="images/syntax/update-stmt.gif"></img> </blockquote>
<h4><a href="syntaxdiagrams.html#qualified-table-name">qualified-table-name:</a></h4><blockquote> <img alt="syntax diagram qualified-table-name" src="images/syntax/qualified-table-name.gif"></img> </blockquote>


<p>An UPDATE statement is used to modify a subset of the values stored in 
zero or more rows of the database table identified by the 
<i>qualified-table-name</i> specified as part of the UPDATE statement.

<p>If the UPDATE statement does not have a WHERE clause, all rows in the
table are modified by the UPDATE. Otherwise, the UPDATE affects only those
rows for which the result of evaluating the WHERE clause expression as a 
<a href="lang_expr.html#booleanexpr">boolean expression is true</a>. It is not an error if the
WHERE clause does not evaluate to true for any row in the table - this just
means that the UPDATE statement affects zero rows.

<p>The modifications made to each row affected by an UPDATE statement are
determined by the list of assignments following the SET keyword. Each
assignment specifies a column name to the left of the equals sign and a
scalar expression to the right. For each affected row, the named columns
are set to the values found by evaluating the corresponding scalar 
expressions. If a single column-name appears more than once in the list of
assignment expressions, all but the rightmost occurence is ignored. Columns
that do not appear in the list of assignments are left unmodified. The scalar
expressions may refer to columns of the row being updated. In this case all
scalar expressions are evaluated before any assignments are made.

<p>The optional conflict-clause allows the user to nominate a specific
constraint conflict resolution algorithm to use during this one UPDATE command.
Refer to the section entitled <a href="lang_conflict.html">ON CONFLICT</a> for additional information.

<h3>Restrictions on UPDATE Statements Within CREATE TRIGGER</h3>

<p>The following additional syntax restrictions apply to UPDATE statements that
occur within the body of a <a href="lang_createtrigger.html">CREATE TRIGGER</a> statement.  

<ul>
  <li><p>The <i>table-name</i> specified as part of an UPDATE statement within
      a trigger body must be unqualified. In other words, the
      <i>database-name</i><b>.</b> prefix on the table name of the UPDATE is
      not allowed within triggers. Unless the table to which the trigger
      is attached is in the TEMP database, the table being updated by the
      trigger program must reside in the same database as it. If the table
      to which the trigger is attached is in the TEMP database, then the
      unqualified name of the table being updated is resolved in the same way
      as it is for a top-level statement (by searching first the TEMP database,
      then the main database, then any other databases in the order they were
      attached).

  <li><p>The INDEXED BY and NOT INDEXED clauses are not allowed on UPDATE
      statements within triggers.</p>

  <li><p>The LIMIT and ORDER BY clauses for UPDATE are unsupported within
      triggers, regardless of the compilation options used to build SQLite.
</ul>

<h3>Optional LIMIT and ORDER BY Clauses</h3>

<p>If SQLite is built with the <a href="compile.html#enable_update_delete_limit">SQLITE_ENABLE_UPDATE_DELETE_LIMIT</a>
compile-time option then the syntax of the UPDATE statement is extended
with optional ORDER BY and LIMIT clauses as follows:</p>

<h4><a href="syntaxdiagrams.html#update-stmt-limited">update-stmt-limited:</a></h4><blockquote> <img alt="syntax diagram update-stmt-limited" src="images/syntax/update-stmt-limited.gif"></img> </blockquote>


<p>If an UPDATE statement has a LIMIT clause, the maximum number of rows that
will be updated is found by evaluating the accompanying expression and casting
it to an integer value. A negative value is interpreted as "no limit".

<p>If the LIMIT expression evaluates to non-negative value <i>N</i> and the
UPDATE statement has an ORDER BY clause, then all rows that would be updated in
the absence of the LIMIT clause are sorted according to the ORDER BY and the
first <i>N</i> updated. If the UPDATE statement also has an OFFSET clause,
then it is similarly evaluated and cast to an integer value. If the OFFSET
expression evaluates to a non-negative value <i>M</i>, then the first <i>M</i>
rows are skipped and the following <i>N</i> rows updated instead.

<p>If the UPDATE statement has no ORDER BY clause, then all rows that
would be updated in the absence of the LIMIT clause are assembled in an
arbitrary order before applying the LIMIT and OFFSET clauses to determine 
which are actually updated.

<p>The ORDER BY clause on an UPDATE statement is used only to determine which
rows fall within the LIMIT. The order in which rows are modified is arbitrary
and is not influenced by the ORDER BY clause.


